NAME

longjmp — non-local goto

SYNOPSIS

#include <setjmp.h>

_Noreturn void longjmp(jmp_buf env, int val);


DESCRIPTION

^[CX] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2024 defers to the ISO C standard.

The longjmp() function shall restore the environment saved by the most recent invocation of setjmp() in the same process, with the corresponding jmp_buf argument. If the most recent invocation of setjmp() with the corresponding jmp_buf occurred in another thread, or if there is no such invocation, or if the function containing the invocation of setjmp() has terminated execution in the interim, or if the invocation of setjmp() was within the scope of an identifier with variably modified type and execution has left that scope in the interim, the behavior is undefined. ^[CX]  It is unspecified whether longjmp() restores the signal mask, leaves the signal mask unchanged, or restores it to its value at the time setjmp() was called.

All accessible objects have values, and all other components of the abstract machine have state (for example, floating-point status flags and open files), as of the time longjmp() was called, except that the values of objects of automatic storage duration are unspecified if they meet all the following conditions:

• They are local to the function containing the corresponding setjmp() invocation.

• They do not have volatile-qualified type.

• They are changed between the setjmp() invocation and longjmp() call.

^[CX] Although longjmp() is an async-signal-safe function, if it is invoked from a signal handler which interrupted a non-async-signal-safe function or equivalent (such as the processing equivalent to exit() performed after a return from the initial call to main()), the behavior of any subsequent call to a non-async-signal-safe function or equivalent is undefined.

The effect of a call to longjmp() where initialization of the jmp_buf structure was not performed in the calling thread is undefined.

RETURN VALUE

After longjmp() is completed, thread execution shall continue as if the corresponding invocation of setjmp() had just returned the value specified by val. The longjmp() function shall not cause setjmp() to return 0; if val is 0, setjmp() shall return 1.

ERRORS

No errors are defined.

EXAMPLES

None.

APPLICATION USAGE

Applications whose behavior depends on the value of the signal mask should not use longjmp() and setjmp(), since their effect on the signal mask is unspecified, but should instead use the siglongjmp() and sigsetjmp() functions (which can save and restore the signal mask under application control).

It is recommended that applications do not call longjmp() or siglongjmp() from signal handlers. To avoid undefined behavior when calling these functions from a signal handler, the application needs to ensure one of the following two things:

1. After the call to longjmp() or siglongjmp() the process only calls async-signal-safe functions and does not return from the initial call to main().

2. Any signal whose handler calls longjmp() or siglongjmp() is blocked during every call to a non-async-signal-safe function, and no such calls are made after returning from the initial call to main().

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

setjmp, sigaction, siglongjmp, sigsetjmp

XBD <setjmp.h>

CHANGE HISTORY

First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 5

The DESCRIPTION is updated for alignment with the POSIX Threads Extension.

Issue 6

Extensions beyond the ISO C standard are marked.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

• The DESCRIPTION now explicitly makes longjmp()'s effect on the signal mask unspecified.

The DESCRIPTION is updated for alignment with the ISO/IEC 9899:1999 standard.

Issue 7

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0365 [394] is applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0202 [516] is applied.

Issue 8

Austin Group Defect 1302 is applied, aligning this function with the ISO/IEC 9899:2018 standard.